Telomerase is a ribonucleoprotein complex responsible for replenishing the G-rich strand of the telomere terminal repeats. It employs a small, embedded segment of a stably associated RNA component as template, and pre-existing chromosomal ends as primers to synthesize telomere tracts. Telomerase activity is specifically activated in cancer cells, and promotes neoplastic transformation by endowing cells with unlimited replicative potential. Inhibition or depletion of telomerase in tumor cells leads to telomere shortening and apoptosis, validating the potential of anti-telomerase strategies in cancer therapy. The goal of this research is to understand telomerase mechanisms and regulation using the budding yeast Saccharomyces cerevisiae as a model system. Our studies during the past funding period have yielded a wealth of information on the structure and functions of the catalytic TERT protein. In addition, they revealed (i) two telomerase-specific domains within the catalytic TERT subunit that are responsible for a unique feature of telomerase enzymology, i.e., the ability to repetitively copy a short RNA template;(ii) an alpha-helical repeat domain (named TPR) in the Est1 p subunit that possesses a novel RNA-binding activity, and (iii) regulation of telomere structure and function by an ATP-dependent chromatin remodeling complex (the INO80 complex). Based on these findings, we propose a series of biochemical, genetic and cell biological studies directed toward (1) clarifying the mechanisms and biological significance of the protein- DNA and protein-RNA interactions mediated by telomerase components, and (2) ascertaining the roles of chromatin remodeling complexes in modulating telomere structures and telomerase activity. Results from the proposed studies are expected to broaden and deepen current understandings of telomerase mechanisms and provide insights into a novel aspect of telomere regulation. Because almost all of the factors that will be examined are conserved between yeast and humans, we anticipate that many of the findings will be applicable to human telomeres and telomerase, and will facilitate the development of clinical applications based on telomere biology.